In manufacturing lenses, and particularly lenses for eyeglasses, the use of plastics is often desirable due to their light weight and durability. Plastic lenses also provide relatively economic vision correction. Methods for producing plastic lenses of various prescriptions are well known.
As used herein a "plastic" lens is one fashioned from optical quality resin materials. Standard industrial materials for use in plastic lenses include mixtures containing allyl diglycol carbonates. Other optical quality resin materials which show promise are polycarbonates of bisphenol A.
Photochromic additives are typically provided by techniques that entail for example, dipping plastic lenses into a heated solution that contains a photochromic additive dissolved in an organic solvent (such as methanol, cyclohexane and methyl ethyl ketone) at a predetermined elevated temperature and for a predetermined time. Many lens materials, however, cannot be provided with photochromic additives using such techniques. For example, CR-39, sold by PPG Industries cannot be used in connection with such techniques. Instead, only a relatively few materials which form soft materials with low cross link density can be used. The most common of such resins is CR-307, which is sold by PPG Industries.
More importantly, the above techniques for providing plastic lenses with photochromic additives are expensive and time-consuming. Such methods are disadvantageous in that substantial quantities of hazardous materials must be purchased, stored and properly disposed of; in that a substantial investment in handling equipment must be made; and in that lenses must be shipped back and forth between photochromic processors and small lensmaking operations (that cannot afford the significant capital investment required for providing photochromic additives using current technology).
Accordingly, there is presently a need in the lensmaking art for a process wherein finished lenses containing photochromic additives can be quickly and inexpensively manufactured by small lensmaking operations. Moreover, there is a need for a method wherein photochromic additives can be used in connection with advantageous optical quality resin materials such as allyl diglycol carbonates.